describe what happens when water boils. Explain the physics of why the boiling t
ID: 1608579 • Letter: D
Question
describe what happens when water boils. Explain the physics of why the boiling temperature changes with the pressure. What determines the physical conditions at which boiling takes place? Cooling takes place during boiling in the sense that heat is leaving in the vapor from the liquid. The process of vaporization is cooling the water by carrying heat away from it. But the liquid is NOT cooling in the common sense that its temperature is falling. It remains at the same temperature, the boiling point, because heat is also flowing into the water from the burner (or whatever is the heat source) at the same rate. There is no net loss of heat and no change of temperature (once the boiling point is reached). Would boiling continue or the temperature immediately change if there were no more heat flowing in? What does the heat flowing into boiling water do? Have you ever noticed how fast a boiling pot of water stops boiling as soon as you turn off the heat source? Why is this?
Explanation / Answer
One thing to note here is that heat is not the only factor that affects boiling. water is held in its liquid state or any other state( to say) by atmospheric pressure. Boiling point changes with pressue in a way that at higher altitudes where pressure is less, vapor pressure needs to be lower to reach boiling point. Water boils at lower temperature at high altitude but this doesnt mean that we can cook faster, it means it will evaporate quickly. so we need to boil cooking materials again and again.
If pressure on the water decreases, the rate of evaporation increases. This is the case at higher altitude. Now when a liquid is heated it eventually reaches a temperature where bubbles will form. This temperature is called boiling point. At this point what happens is, temperature will remain constant( 100 degree celsius in case of water) until all the liquid is converted to vapor form. Temperature will not change.
when we heat water in a pot, the actual tempearture of pot will be above 100 when water is at 100. Pot of water stops boiling when there is suffuceint phase change. but before that there will be a change from steady state to a new steady state where steam will be less hot which cools more rapidly thus slowly cooling down the pot. atmospheric pressure also plays a role in this. it tries to maintain equilibrium. Therefore the time to cool down totally depends on transition from one steady state (during heat supply) to other state(when heat is cut off).
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